Analog to Digital Converters (ADCs) are often preceded by a multiplexer (MUX), to allow analog signals from several different signal channels or lines to be multiplexed into the ADC for sampling and digital conversion. The non-linear capacitance of the off channels in the MUX (often as many as 15 or 31) can be a significant source of distortion, as well as the distortion due to non-linear on resistance of the switches in the MUX of the channel that is presently on. In this latter respect, when integrated with an ADC the MUX switches of each MUX channel will commonly take the form of transmission gates (“TG”) which comprise a PMOS transistor in parallel with an NMOS transistor.
In use, when a “high” input voltage (Vin close to Vdd) is applied to the switch, there is little gate-source voltage (Vgs) across the NMOS transistor and it is largely switched off. However, under such circumstances there is a good gate-source voltage across the PMOS transistor, and it is fully conducting. Conversely, when a low (Vin close to Vss) input voltage is applied, there is little Vgs across the PMOS device and it is switched off, but conversely the NMOS device is driven hard on.
However, when the input voltage is mid range, both transistors may be conducting, but not to their lowest impedance state. Consequently the impedance of the TG switch may be at its largest “on” value (“Ron”). This non-linear Ron characteristic leads to distortion in the signal passing through the on channel of the MUX to the ADC with further distortion also being introduced by parasitic capacitances of the MUX channels.